Light emitting device and fabricating method thereof

ABSTRACT

A light-emitting device includes a light-emitting element for emitting primary light, and a wavelength conversion unit for absorbing part of the primary light and emitting secondary light having a wavelength longer than that of the primary light, wherein the wavelength conversion unit includes plural kinds of phosphors having light absorption characteristics different from each other, and then at least one kind of phosphor among the plural kinds of phosphors has an absorption characteristic that can absorb the secondary light emitted from at least another kind of phosphor among the plural kinds of phosphors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/774,495, filed May 5, 2010, which is a continuation of U.S. patentapplication Ser. No. 11/442,000, filed on May 26, 2006, now U.S. Pat.No. 7,737,621, issued on Jun. 15, 2010, which is based on JapanesePatent Applications Nos. 2005-157056, 2005-202851 and 2006-059689 filedwith the Japan Patent Office on May 30, 2005, Jul. 12, 2005 and Mar. 6,2006, respectively, the entire contents of which are hereby incorporatedby reference.

FIELD OF THE INVENTION

The present invention relates to improvement in a light-emitting deviceor a white light-emitting device including a semiconductorlight-emitting element for emitting primary light and a wavelengthconversion unit containing phosphor for absorbing the primary light andthen emitting secondary light.

BACKGROUND OF THE INVENTION

The light-emitting device or the white light-emitting device includingthe semiconductor light-emitting element and the wavelength conversionunit has attracted attention as a light-emitting device or a whitelight-emitting device of next generation expected to realize low powerconsumption, downsizing, high luminance, and wide range of colorreproducibility, for which research and development have been conductedvigorously.

A GaN-based light-emitting element, a ZnO-based light-emitting element,or the like is used as the semiconductor light-emitting element. Lighthaving a wavelength in the range from the longer-wavelength side ofultraviolet to blue (i.e., about 380 nm to about 480 nm) is used as theprimary light emitted from the light-emitting element. The wavelengthconversion units have been proposed, which use various kinds ofphosphors suitable for converting the primary light to the secondarylight.

An example for the light-emitting device having such a wavelengthconversion function is disclosed in Japanese Patent Laying-Open No.2004-071357. This document describes use of InN-based nano-crystals asphosphor but gives no detailed explanation for light emissioncharacteristics thereof. Japanese Patent Laying-Open No. 2004-179644discloses a light-emitting device taking account of light diffusionefficiency in each of plural phosphor layers that include lightdiffusing agents added in resin. In this document as well, there is nodetailed explanation for light emission characteristics of the phosphorlayers.

Meanwhile, in the field of application of the white light-emittingdevice, its use as a white lighting device for a backlight in a liquidcrystal display (LCD) (especially in a liquid crystal television (TV)set) or for a flashlight for a camera is most important. The whitelighting device is required to have high luminous efficiency and, at thesame time, good color reproducibility. Particularly, it is preferablethat the white lighting device is able to give white light analogous toblackbody radiation that is colorless.

Japanese Patent Laying-Open Nos. 2004-071357 and 2004-179644 give nodetailed explanation for light emission characteristics regarding pluralkinds of phosphors included in the light-emitting device and notechnical consideration for effect of relative arrangement order of theplural kinds of phosphors.

From the standpoint of preventing the resin layers containing thephosphors from being deteriorated by ultraviolet light during theoperating period of the light-emitting device, in the case of using alight-emitting element having an emission peak wavelength in the rangeof 400 nm to 500 nm corresponding to the wavelength range of blue, theexcitation characteristics in the visible light range in the wavelengthconversion unit (i.e., in the plural kinds of phosphors containedtherein) suitable for the wavelength of the primary light have thecritical meaning.

In other words, in order to exert the characteristics of the wavelengthconversion unit containing plural kinds of phosphors to the maximumextent in a light-emitting device, it is important to configure anoptimal stacked-layer state of the phosphors taking into considerationthe excitation characteristics of the respective phosphors.

On the other hand, in order to enhance color reproducibility of thewhite light-emitting device, it is desirable to use a light sourceexhibiting high spectral purity for each of three primary colors of red,green and blue. It is conceivable to use an LED (light-emitting diode)or a semiconductor laser, for example, for each color light source.However, it is not possible to obtain a semiconductor light sourcehaving good luminous efficiency of green. Further, the luminance of eachof the semiconductor light sources needs to be controlled independentlyfrom each other, which requires a drive circuit of a large scale.

As a method of implementing a white light source using one kind ofsemiconductor light source, Japanese Patent Laying-Open No. 10-242513discloses a method of using a combination of a blue LED and a phosphorthat is excited by the blue light and emits yellow light. With thismethod, however, there is a problem that the color reproducibility ispoor, since the spectral purity of green is low, in addition toinsufficient red component.

Similarly, there have been proposed various methods in each of which anLED for emitting light having a wavelength in the range from ultravioletlight of relatively longer wavelength to blue (i.e., from 380 nm to 480nm) is combined with a phosphor that emits light of blue, green or redin response to the primary light emitted from the LED. For example,Japanese Patent Laying-Open No. 2004-327492 discloses a method of usinga phosphor only for green light for the purpose of enhancing luminousefficiency of green. The method disclosed in this document, however,still requires a drive circuit of a large scale, since it uses pluralkinds of semiconductor light-emitting elements.

Japanese Patent Laying-Open No. 2002-171000 discloses a method ofcombining an LED for emitting ultraviolet light and plural kinds ofphosphors that emit lights of blue, green and red in response to theprimary light emitted from the LED. The document however gives nosuggestion about an optimal combination of the LED and the plural kindsof phosphors, particularly about the combination for producing a lightsource able to be regarded as blackbody radiation and at the same timefor achieving high luminous efficiency and good color reproducibilityover a wide range. In particular, with the phosphors disclosed inJapanese Patent Laying-Open No. 2002-171000, luminous efficiency isinsufficient especially in the phosphor for emitting red light.

At present, a white light-emitting device mainly includes alight-emitting element for emitting blue light in combination with atrivalent cerium-activated (Y,Gd)₃(Al,Ga)₅O₁₂ phosphor or a divalenteuropium-activated (Sr,Ba,Ca)₂SiO₄ phosphor for being excited by theblue light and then emitting yellow light.

Such a white light-emitting device, however, is not suitable for use asa backlight of a large LCD (especially for a LCD-TV), since its colorreproducibility (NTSC ratio) is a little under 50%. That is, althoughthe white light-emitting device having the combination of asemiconductor light-emitting element and a phosphor is advantageous inthat it does not include mercury and can achieve low power consumption,downsizing and high luminance as compared to the cold cathodefluorescent lamp mainly used at present, there is an urgent need forimprovement of its color reproducibility (NTSC ratio).

Here, the NTSC ratio is normalization with respect to an area of atriangle formed by connecting the chromaticity coordinates (x, y) of red(0.670, 0.330), green (0.210, 0.710) and blue (0.140, 0.080) in the XYZcolor system chromaticity diagram defined by the National TelevisionSystem Committee (NTSC).

Japanese Patent Laying-Open No. 2003-121838 discloses a prior art thathas focused on color reproducibility (NTSC ratio) in the LCD. Thisdocument describes that a backlight light source has a spectral peak inthe range from 505 nm to 535 nm; that an activator in a phosphor foremitting green light used in the light source includes europium,tungsten, tin, antimony, or manganese; and that MgGa₂O₄:Mn andZn₂SiO₄:Mn are used as the phosphors for emitting green light in theembodiment. In the case that the peak wavelength of the light-emittingelement is in the range from 380 nm to 450 nm, however, it cannot besaid that every phosphor containing europium, tungsten, tin, antimony,or manganese can suitably be used. More specifically, MgGa₂O₄:Mn andZn₂SiO₄:Mn mentioned in the embodiment of Japanese Patent Laying-OpenNo. 2003-121838 each have their very low luminous efficiency with theexcitation light in the range from 380 nm to 450 nm, and thus thesephosphors are not suitable for use in the present invention.

Further, Japanese Patent Laying-Open No. 2004-287323 describes that notonly an RGB (red, green, blue)-LED having LED chips for emitting red,green and blue lights, respectively, contained in a package, but also atri-color type fluorescent tube, an ultraviolet LED+RGB phosphors, anorganic EL light source, and the like can be used as a backlight. Inthis document, however, there is no specific description regarding theRGB phosphors in the ultraviolet LED+RGB phosphors.

SUMMARY OF THE INVENTION

In view of the above-described state of the prior art, an object of thepresent invention is to provide a light-emitting device that includes awavelength conversion unit containing plural kinds of phosphors foremitting light with high efficiency in response to light in thewavelength range from 380 nm to 450 nm emitted from a semiconductorlight-emitting element, wherein it is possible to readily set color oflight to be emitted from the device and also achieve high luminance.Another object of the present invention is to provide a whitelight-emitting device having excellent color reproducibility (NTSCratio).

A light-emitting device according to an aspect of the present inventionincludes: a light-emitting element for emitting primary light; and awavelength conversion unit for absorbing part of the primary light andthen emitting secondary light having a wavelength greater than that ofthe primary light; wherein the wavelength conversion unit includesplural kinds of phosphors different in light absorption characteristicfrom each other, and at least one kind of phosphor among the pluralkinds of phosphors has an absorption characteristic that can absorb thesecondary light emitted by at least another kind of phosphor among theplural kinds of phosphors. With this light-emitting device, it ispossible to readily set color of light to be emitted from the device andalso achieve high luminance.

Preferably, a phosphor for emitting light of a relatively longerwavelength among the plural kinds of phosphors is arranged closer to thelight-emitting element. With this configuration, a color light emittedfrom each of the plural kinds of phosphors toward the outside of thelight-emitting device can efficiently be extracted to the outside of thedevice, without being absorbed again by the other phosphor(s) thatis/are for emitting the other color light(s) and arranged on the outerside. Further, light emitted inward contributes to excitation of thephosphor(s) that is/are for emitting light(s) of longer wavelength(s)and arranged on the inner side. Accordingly, emission loss in thelight-emitting device as a whole can be restricted to the minimum level.

The light-emitting element used in the present invention can be producedusing a gallium nitride-based semiconductor.

The phosphor for emitting blue light used in the present invention mayinclude at least one kind selected from a divalent europium-activatedhalophosphate phosphor represented by (M1,Eu)₁₀(PO₄)₆.Cl₂, a divalenteuropium-activated aluminate phosphor represented by a(M2,Eu)O.bAl₂O₃,and a divalent europium-manganese co-activated aluminate phosphorrepresented by a(M2,Eu_(c),Mn_(d))O.bAl₂O₃, where M1 represents at leastone kind of element selected from Mg, Ca, Sr and Ba; M2 represents atleast one kind of element selected from Mg, Ca, Sr, Ba and Zn; and a, b,c, and d represent numbers satisfying a>0, b>0, 0.1≦a/b≦1.0, and0.001≦d/c≦0.2.

The phosphor for emitting green light used in the present invention mayinclude at least one kind selected from a divalent europium-manganeseco-activated aluminate phosphor represented bya(M2,Eu_(e),Mn_(f))O.bAl₂O₃, a divalent europium-activated silicatephosphor represented by 2(M1_(1-g),Eu_(g))O.SiO₂, and a trivalentcerium-activated silicate phosphor represented byMI₃(MII_(1-h),Ce_(h))₂(SiO₄)₃, where M2 represents at least one kind ofelement selected from Mg, Ca, Sr, Ba and Zn; a, b, e, and f representnumbers satisfying a>0, b>0, 0.1≦a/b≦1.0, and 0.3≦f/e≦5.0; M1 representsat least one kind of element selected from Mg, Ca, Sr and Ba; grepresents a number satisfying 0.005≦g≦0.10; MI represents at least onekind of element selected from Mg, Ca, Sr and Ba; MII represents at leastone kind of element selected from Al, Ga, In, Sc, Y, La, Gd and Lu; andh represents a number satisfying 0.005≦h≦0.5.

The phosphor for emitting red light used in the present invention mayinclude a divalent europium-activated nitride phosphor represented by(MIII_(1-j),Eu_(j))MIVSiN₃, where MIII represents at least one kind ofelement selected from Mg, Ca, Sr and Ba; MIV represents at least onekind of element selected from Al, Ga, In, Sc, Y, La, Gd and Lu; and jrepresents a number satisfying 0.001≦j≦0.05.

A light-emitting device according to another aspect of the presentinvention includes: a light-emitting element for emitting primary light;and a wavelength conversion unit for absorbing at least part of theprimary light and emitting secondary light having a wavelength longerthan that of the primary light; wherein the wavelength conversion unitincludes at least a phosphor for emitting green light and a phosphor foremitting red light, and the phosphor for green light includes a divalenteuropium-manganese co-activated aluminate phosphor represented bya(M2,Eu_(e),Mn_(f))O.bAl₂O₃, where M2 represents at least one kind ofelement selected from Mg, Ca, Sr, Ba and Zn; and a, b, e and f representnumbers satisfying a>0, b>0, 0.1≦a/b≦1.0 and 0.3≦f/e≦5.0.

In this light-emitting device, the phosphor for green light has a narrowemission spectrum, so that good color reproducibility can be obtainedusing one semiconductor light-emitting element. More specifically, Euserving as the activator absorbs the primary light sufficiently butitself hardly emits light. The absorbed energy is transferred to Mn, andthen green light is generated as emission from Mn. Due to the narrowemission spectrum from Mn, green color having a narrow spectral width isobtained with high efficiency.

Further, in the case that Sr is added to the base crystal of thephosphor, the primary light having even a relatively longer wavelengthcan also serve as the excitation light. This can prevent the mold resincontaining the phosphor from deteriorating due to radiation ofshort-wavelength light, and thus the lifetime of the light-emittingdevice can be elongated.

Preferably, the phosphor for red light includes a divalenteuropium-activated nitride phosphor represented by(M3_(1-g)Eu_(g))M4SiN₃, where M3 represents at least one kind of elementselected from Mg, Ca, Sr and Ba; M4 represents at least one kind ofelement selected from Al, Ga, In, Sc, Y, La, Gd and Lu; and g representsa number satisfying 0.001≦g≦0.05. With this phosphor for emitting redlight, red light of a narrow spectral width can be obtained with highefficiency.

The wavelength conversion unit may further include a phosphor foremitting blue light, which contains at least one kind selected from adivalent europium-activated halophosphate phosphor represented by(M1,Eu)₁₀(PO₄)₆.Cl₂, a divalent europium-activated aluminate phosphorrepresented by a (M2,Eu)O.bAl₂O₃, and a divalent europium-manganeseco-activated aluminate phosphor represented bya(M2,Eu_(e),Mn_(d))O.bAl₂O₃, where M1 represents at least one kind ofelement selected from Mg, Ca, Sr and Ba; M2 represents at least one kindof element selected from Mg, Ca, Sr, Ba and Zn; and a, b, c and drepresent numbers satisfying a>0, b>0, 0.1≦a/b≦1.0, and 0.001≦d/c≦0.2.With this phosphor for emitting blue light, blue light of a narrowspectral width can be obtained with high efficiency.

Preferably, the light-emitting element is formed of a galliumnitride-based semiconductor, and the primary light has a peak wavelengthin the range from 380 nm to 450 nm. More preferably, the peak wavelengthof the primary light is in the range from 390 nm to 420 nm. Such asemiconductor light-emitting element can efficiently excite the threekinds of phosphors for emitting blue, green and red lights.

Wavelength components of the primary light emitted from thelight-emitting element and wavelength components of the secondary lightemitted from each of the phosphors for emitting red light, green lightand blue light can be adjusted such that the light emitted from thelight-emitting device has chromaticity that is regarded as blackbodyradiation at a prescribed color temperature. By such adjustment, thelight-emitting device can emit natural (white) light and thus it can beused as a light source for illumination of good quality.

Preferably, the light-emitting element is covered with a plurality ofresin layers each of which contains one of the phosphors for emittingred light, green light and blue light, and the phosphor contained in theresin layer closer to the light-emitting element emits the secondarylight having a longer peak wavelength as compared to the phosphorcontained in the resin layer farther from the light-emitting element.With this arrangement, the light emitted from the phosphor in the resinlayer arranged closer to the light-emitting element is prevented frombeing absorbed in the phosphor within the resin layer arranged fartherfrom the light-emitting element, whereby making it possible to obtain awhite light-emitting device of high efficiency. It is also possible todesign a white light-emitting device based on a result of measuredluminous efficiency with respect to thickness of every resin layercontaining a single kind of phosphor.

Preferably, the light-emitting element is molded with a plurality ofresin layers that are stacked one on another such that each layer has apredetermined thickness. This can facilitate fabrication of the whitelight-emitting device.

The light-emitting device according to the present invention can befabricated by a method including the steps of: mounting a light-emittingelement on a bottom surrounded by reflective inner wall surfaces in apackage; introducing and setting a first liquid resin kneaded with afirst kind of phosphor to cover the light-emitting element mounted inthe package; and introducing and setting a second liquid resin kneadedwith a second kind of phosphor on the first resin thus set and on thereflective surfaces, wherein the second kind of phosphor is for emittingsecondary light having a shorter wavelength as compared with the firstkind of phosphor.

Further, the light-emitting device according to the present inventionmay be fabricated by another method including the steps of: mounting alight-emitting element on a bottom surrounded by reflective inner wallsurfaces in a package; introducing a liquid resin kneaded with pluralkinds of phosphors having different median values of particle sizes tocover the light-emitting element mounted in the package; and leaving theintroduced liquid resin at rest for a prescribed time period to causethe phosphors to settle such that a particular kind of phosphor having agreater median value of particle size among the plural kinds ofphosphors has a greater concentration near the light-emitting element.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic vertical cross sectional view showing a structureof a light-emitting device according to an embodiment of the presentinvention.

FIG. 2 is a graph showing spectral distribution of excitation andemission of a phosphor for emitting red light.

FIG. 3 is a graph showing spectral distribution of excitation andemission of a phosphor for emitting green light.

FIG. 4 is a graph showing spectral distribution of excitation andemission of a phosphor for emitting blue light.

FIG. 5 is a graph showing emission spectral distribution in alight-emitting device according to an embodiment of the presentinvention.

FIG. 6 is a graph showing spectral characteristics of color filters usedfor evaluation of properties of the light-emitting device.

FIG. 7 shows schematic cross sectional views illustrating a fabricationprocess of a light-emitting device according to another embodiment ofthe present invention.

FIG. 8 is an xy chromaticity diagram showing the relation between thetemperature of blackbody radiation and the chromaticity coordinates.

FIG. 9 is a schematic vertical cross sectional view showing a structureof a light-emitting device according to a further embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, a light-emitting device according to Example 1 of the presentinvention is illustrated in a schematic vertical cross sectional view.This light-emitting device 10 includes a light-emitting element 11 foremitting primary light, and a wavelength conversion unit 12 that absorbsat least part of the primary light and emits secondary light having awavelength longer than that of the primary light. Light-emitting element11 is mounted on a cathode terminal 18 and it is electrically connectedto an anode terminal 17 and cathode terminal 18 via gold wires 19.

For light-emitting element 11, it is possible to use a gallium nitride(GaN)-based light-emitting diode having an emission peak wavelength at410 nm, for example.

Wavelength conversion unit 12 includes a layer 13 that contains aphosphor having a composition of (Ca_(0.98)Eu_(0.02))AlSiN₃ for emittingred light, a layer 14 that contains a phosphor having a composition of(Ba_(0.85)Eu_(0.15)) (Mg_(0.80)Mn_(0.20))Al₁₀O₁₇ for emitting greenlight, and a layer 15 that contains a phosphor having a composition of(Ba_(0.80)Eu_(0.20))MgAl₁₀O₁₇ for emitting blue light.

FIG. 2 shows spectral distribution of excitation and emission of thephosphor for emitting red light used in Example 1; FIG. 3 shows spectraldistribution of excitation and emission of the phosphor for emittinggreen light; and FIG. 4 shows spectral distribution of excitation andemission of the phosphor for emitting blue light. Specifically, in eachgraph shown in FIGS. 2-4, a horizontal axis represents the wavelength(nm) of light, and a vertical axis represents the relative intensity oflight. Further, curves 31, 33 and 35 in the graphs represent spectraldistribution of excitation of the phosphors, and curves 32, 34 and 36represent spectral distribution of emission from the phosphors.

As seen from FIGS. 2-4, each phosphor in Example 1 can very efficientlyemits red, green or blue light in response to the primary light havingthe peak wavelength of 410 nm.

Further, it is recognized that the phosphor for emitting red light canemit the red light by absorbing even the green light (wavelength around520 nm) and the blue light (wavelength around 450 nm) emitted from thephosphors for emitting green and blue lights, respectively.

However, it is not beneficial to excite the phosphor for emitting redlight by the green light (near 520 nm wavelength) and the blue light(near 450 nm wavelength) emitted as the secondary lights from thephosphors excited by the primary light, since it would decreasewavelength conversion efficiency as a whole.

More specifically, to obtain a light-emitting device of high luminance,it is important to stack phosphor layer 13 for red light, phosphor layer14 for green light and phosphor layer 15 for blue light in this order,as in this Example 1. It is noted that the phosphor for green light andthe phosphor for blue light may be mixed to form a single light-emittinglayer, since the phosphor for green light cannot be excited by the bluelight to emit green light.

In Example 1, each of the phosphor for red light, the phosphor for greenlight and the phosphor for blue light is kneaded into a (silicone-basedor epoxy-based) binder resin, which is then introduced into a cup 16 ofthe device package shown in FIG. 1 in order of red light-emitting layer13, green light-emitting layer 14 and blue light-emitting layer 15,which are then set to form a phosphor layered structure 12.

On the inner wall of cup 16, terraces 16 a are provided so as to preventthe resins kneaded with the phosphors and introduced into the cup fromclimbing up by surface tension along the inner wall of the cup, therebymaking it possible to obtain a uniform thickness for each light-emittinglayer. Further, it is preferable that the inner wall of the cup is madeto be inclined more steeply in the upper part than in the lower partwith the boundary formed by terrace 16 a. This can reduce shade ofterrace 16 a, which is caused in the upper phosphor layer by lightemitted from the lower phosphor layer.

As Comparative Example 1, phosphors of the exactly same kinds as inExample 1 were mixed at a mass ratio of phosphor for blue light:phosphorfor green light:phosphor for red light=2.0:1.5:1.0, to form a singlelight-emitting layer serving as the wavelength conversion unit.

Table 1 shows evaluation results of white light emitted from thelight-emitting devices of Example 1 and Comparative Example 1.

TABLE 1 Brightness (relative value) Tc-duv Example 1 100% 7000 K+0.001Comparative Example 1 70% 7000 K+0.001

As seen from Table 1, while the light-emitting devices of Example 1 andComparative Example 1 emit light of the same color, the light-emittingdevice of Example 1 is considerably improved in brightness as comparedto Comparative Example 1.

Here, “Tc” represents the correlated color temperature of light emittedfrom the light-emitting device, and “duv” represents the deviation ofthe chromaticity point of the emitted light from the blackbody radiationlocus (i.e., the length of the normal drawn from the chromaticity pointof the emitted light down to the blackbody radiation locus on the U*V*Wchromaticity diagram (CIE 1964 uniform color space)). It is generallyconsidered that light having duv of less than 0.01 is felt as natural(white) light, similarly to light emitted from a common tungstenfilament light bulb and the like. With the blackbody radiationtemperature of 7000K, natural white light is obtained, since it is closeto the color temperature of the sun.

In each of Example 1 and Comparative Example 1, although the primarylight emitted from the light-emitting element has its peak wavelength of410 nm in the wavelength range effective for excitation of thephosphors, it is low in luminous efficacy and thus does not contributeto brightness of the light-emitting device even when it is leakedoutside. Therefore, it is preferable that a sheet 12 a coated with anoptical film (for example, multi-layered interference film) having theproperty of reflecting only the primary light is laminated on theoutermost surface of phosphor layer structure 12. The primary lightreflected by this sheet 12 a can again contribute to excitation of thephosphor layers, thereby enabling emission of brighter light from thelight-emitting device as a whole.

In Example 2 of the present invention, a gallium nitride (GaN)-basedlight-emitting diode having a peak wavelength at 460 nm was used as thelight-emitting element.

For the wavelength conversion unit, there were used a phosphor for greenlight represented by a composition of (Sr_(0.75)Ba_(0.24)Eu_(0.01))₂SiO₄and a phosphor for red light represented by a composition of(Ca_(0.985)Eu_(0.015))AlSiN₃. It is recognized from FIG. 2 that thephosphor for red light can absorb green light (wavelength near 550 nm)emitted from the phosphor for green light and then can emit red light.

The light-emitting device of Example 2 was fabricated using thelight-emitting diode and the phosphors, in a similar manner as inExample 1. As Comparative Example 2, there was fabricated alight-emitting device having a wavelength conversion unit formed of asingle light-emitting layer made by mixing exactly the same kinds ofphosphors as in Example 2.

Properties of the light-emitting devices of Example 2 and ComparativeExample 2 were evaluated, of which results are shown in Table 2.

TABLE 2 Brightness (relative value) Tc-duv Example 2 100% 6000 K−0.001Comparative Example 2 75% 6000 K−0.001

As seen from Table 2, while the light-emitting device of Example 2 emitslight having the same color as that emitted from the light-emittingdevice of Comparative Example 2, its brightness is considerablyimproved.

Light-emitting devices of Examples 3-8 and Comparative Examples 3-8 werefabricated in a similar manner as in Example 1 and Comparative Example1, except that the emission peak wavelengths of the light-emittingelements and the kinds of phosphors used were changed. Evaluationresults of their properties are shown in Table 3.

TABLE 3 Peak wavelength Brightness of primary light Phosphors (relativevalue) Tc-duv Ex. 3 410 nm red: (Ca_(0.94)Sr_(0.05)Eu_(0.01))AlSiN₃ 100%6500 K green: (Sr_(0.70)Ba_(0.28)Eu_(0.02))₂SiO₄ −0.003 blue:(Sr_(0.74)Ba_(0.20)Ca_(0.05)Eu_(0.01))₁₀(PO₄)₆•Cl₂ Com. Ex. 3 ″ red:(Ca_(0.94)Sr_(0.05)Eu_(0.01))AlSiN₃ 73% 6500 K green:(Sr_(0.70)Ba_(0.28)Eu_(0.02))₂SiO₄ −0.003 blue:(Sr_(0.74)Ba_(0.20)Ca_(0.05)Eu_(0.01))₁₀(PO₄)₆•Cl₂ Ex. 4 410 nm red:(Ca_(0.99)Eu_(0.01))(Al_(0.90)Ga_(0.10))SiN₃ 100% 7500 K green:(Ba_(0.86)Eu_(0.14))(Mg_(0.75)Mn_(0.25))Al₁₀O₁₇ +0.001 blue:(Ba_(0.86)Eu_(0.14))(Mg_(0.99)Mn_(0.01))Al₁₀O₁₇ Com. Ex. 4 ″ red:(Ca_(0.99)Eu_(0.01))(Al_(0.90)Ga_(0.10))SiN₃ 70% 7500 K green:(Ba_(0.86)Eu_(0.14))(Mg_(0.75)Mn_(0.25))Al₁₀O₁₇ +0.001 blue:(Ba_(0.86)Eu_(0.14))(Mg_(0.99)Mn_(0.01))Al₁₀O₁₇ Ex. 5 450 nm red:(Ca_(0.98)Eu_(0.02))AlSiN₃ 100% 5800 K green:(Sr_(0.72)Ba_(0.25)Ca_(0.01)Eu_(0.02))₂SiO₄ −0.002 Com. Ex. 5 ″ red:(Ca_(0.98)Eu_(0.02))AlSiN₃ 75% 5800 K green:(Sr_(0.72)Ba_(0.25)Ca_(0.01)Eu_(0.02))₂SiO₄ −0.002 Ex. 6 450 nm red:(Ca_(0.97)Ba_(0.01)Eu_(0.02))(Al_(0.99)In_(0.01))SiN₃ 100% 7500 K green:Mg₃(Al_(0.85)Ce_(0.15))₂(SiO₄)₃ −0.003 Com. Ex. 6 ″ red:(Ca_(0.97)Ba_(0.01)Eu_(0.02))(Al_(0.99)In_(0.01))SiN₃ 70% 7500 K green:Mg₃(Al_(0.85)Ce_(0.15))₂(SiO₄)₃ −0.003 Ex. 7 465 nm red:(Ca_(0.98)Eu_(0.02))AlSiN₃ 100% 4200 K green:Ca₃(Y_(0.80)Al_(0.10)Ce_(0.10))₂(SiO₄)₃ +0.002 Com. Ex. 7 ″ red:(Ca_(0.98)Eu_(0.02))AlSiN₃ 72% 4200 K green:Ca₃(Y_(0.80)Al_(0.10)Ce_(0.10))₂(SiO₄)₃ +0.002 Ex. 8 460 nm red:(Ca_(0.98)Eu_(0.02))AlSiN₃ 100% 3500 K green:(Sr_(0.76)Ba_(0.22)Eu_(0.02))₂SiO₄ −0.002 Com. Ex. 8 ″ red:(Ca_(0.98)Eu_(0.02))AlSiN₃ 75% 3500 K green:(Sr_(0.76)Ba_(0.22)Eu_(0.02))₂SiO₄ −0.002

As seen from Table 3, the light-emitting devices of Examples 3-8 of thepresent invention are considerably improved in brightness as compared toComparative Examples 3-8, respectively.

FIG. 1 may be referred to also for a white light-emitting deviceaccording to Example 9 of the present invention. A white light-emittingdevice 10 of Example 9 includes a light-emitting element 11 for emittingprimary light, and a wavelength conversion unit 12 that absorbs at leastpart of the primary light and emits secondary light having a wavelengthlonger than that of the primary light. For light-emitting element 11, itis possible to use a gallium nitride (GaN)-based light-emitting diodehaving a peak wavelength at 410 nm.

For wavelength conversion unit 12, a resin layer 13 that contains aphosphor for red light represented by a composition of(Ca_(0.98)Eu_(0.02))AlSiN₃, a resin layer 14 that contains a phosphorfor green light represented by a composition of (Ba_(0.85)Eu_(0.15))(Mg_(0.80)Mn_(0.20))Al₁₀O₁₇, and a resin layer 15 that contains aphosphor for blue light represented by a composition of(Ba_(0.80)Eu_(0.20))MgAl₁₀O₁₇ were stacked successively in this order,with the three kinds of phosphors being contained at a mass ratio of1:1:1.

FIG. 5 is a graph showing emission spectral distribution of the whitelight-emitting device of Example 9. Specifically, in the graph, ahorizontal axis represents emission wavelength (nm), and a vertical axisrepresents radiant intensity (a.u.: arbitrary unit) of light. It isfound in this graph that the light from the phosphor for emitting greenlight (peak wavelength around 520 nm) as the secondary light has anarrow spectral width corresponding to the light emitted from Mn.

In the white light-emitting device of Example 9, brightness and colorreproducibility (NTSC ratio) were evaluated. FIG. 6 shows the spectralcharacteristics of the color filters of blue, green and red that wereused for measurement of the color reproducibility. In this graph, ahorizontal axis represents the wavelength (nm) of light, and a verticalaxis represents the amount (a.u.) of transmitted light. Further, curves37, 38 and 39 in the graph represent the characteristics of the bluefilter, the green filter and the red filter, respectively.

For the purpose of comparison with Example 9, a light-emitting device ofComparative Example 9 was fabricated. The light-emitting device ofComparative Example 9 differs from that of Example 9 only in that asingle resin layer was used for the wavelength conversion unit, whichcontains a phosphor for blue light represented by a composition of(Ba_(0.80)Eu_(0.20))MgAl₁₀O₁₇ and a phosphor for yellow lightrepresented by a composition of(Sr_(0.02)Ba_(0.05)Ca_(0.01)Eu_(0.02))₂SiO₄ mixed at a mass ratio of1.5:1.0.

Table 4 shows evaluation results of the white light emitted by thelight-emitting devices of Example 9 and Comparative Example 9.

TABLE 4 Brightness Color reproducibility (relative value) Tc-duv (NTSCratio) Ex. 9 100.0% 6600 K+0.007 85.0% Com. Ex. 9  97.3% 6600 K+0.00748.1%

As seen from Table 4, the light-emitting device of Example 9 isconsiderably improved not only in brightness but also in colorreproducibility (NTSC ratio) as compared to Comparative Example 9.

In Table 4, the blackbody radiation temperature of 6600K, which is closeto the temperature of the sun, means that natural white light can beobtained.

For the purpose of reference, an xy chromaticity diagram of FIG. 8 showsthe relation between the blackbody radiation temperature and the xychromaticity coordinates. In this chromaticity diagram, a plurality ofcircles represent the spectrum locus, and a plurality of trianglesrepresent the blackbody radiation locus.

The light-emitting device of Example 9, for which FIG. 1 may be referredto, is fabricated more specifically as follows. GaN-based light-emittingelement 11 is mounted on one of a pair of lead frames (thin metalplates) 17, 18, and is electrically connected to lead frames 17, 18 viaa pair of wires 19.

A bowl-shaped cup 16 is formed surrounding light-emitting element 11 byusing a resin of white color having high reflectivity for visible light.On the inner wall of the bowl-shaped cup, terraces 16 a are provided tostabilize the levels of the liquid resins containing the phosphors. As aresult, each of the resin layers 13, 14 and 15 containing the phosphorscan have approximately a predetermined uniform thickness.

Light-emitting element 11 is molded with a resin layer 13 added with aphosphor for secondary light of red, a resin layer 14 added with aphosphor for secondary light of green, and a resin layer 15 added with aphosphor for secondary light of blue, successively in this order. Aftermolding of a resin layer added with one kind of phosphor, it may be setprovisionally before introducing the next resin layer. This can suppressreduction of production efficiency while preventing the adjacent resinlayers from being mixed with each other.

Although the inner wall of cup 16 may be left with the white color resinmaterial, it is more preferable to coat the inner wall with a metalhaving high reflectivity for visible light, such as silver, aluminum orthe like, in order to further improve the luminous efficiency of thelight-emitting device.

In FIG. 7, an alternative fabrication process of a light-emitting devicerelated to Example 9 is illustrated in schematic cross sections.

In FIG. 7(A), GaN-based light-emitting element 11 is mounted on a hardwiring-board 21, and electrically connected thereto via wire 22. Asshown in FIG. 7(B), light-emitting element 11 is molded successivelywith a resin layer added with a phosphor for secondary light of red, aresin layer added with a phosphor for secondary light of green, and aresin layer added with a phosphor for secondary light of blue, and thusis covered with a resin dome 23 containing these resin layers.

Resin dome 23 can be formed without use of a die, a metallic mold or thelike, by using a resin having high thixotropy. As shown in FIG. 7(C),however, a die assembly may be used to accurately regulate thethicknesses of the resin layers contained in resin dome 23.

In FIG. 7(C), light-emitting element 11 is covered with resin layer 23 afor red light that contains the phosphor for emitting red light. Resinlayer 23 a is provisionally set before being pressed by a press mold 25.This allows the thickness t of resin layer 23 a above the top face oflight-emitting element 11 to be a predetermined value, as shown in FIG.7(D). Similarly, resin layer 23 b for green that contains the phosphorfor emitting green light and resin layer 23 c for blue that contains thephosphor for emitting blue light are formed using press mold 25, wherebymaking it possible to obtain a white light-emitting device includinglight-emitting element 11 covered with three resin layers 23 a, 23 b and23 c having the controlled thicknesses, as shown in FIG. 7(E). In thiscase as well, it is possible to suppress reduction of productionefficiency while preventing the adjacent resin layers from being mixedwith each other, because light-emitting element 11 is covered with onekind of resin layer which is then provisionally set before being coveredwith the next resin layer.

In Example 10 of the present invention, a gallium nitride (GaN)-basedlight-emitting diode having a peak wavelength of 390 nm was used as thelight-emitting element. For the wavelength conversion unit, there wereused a resin layer that contains a phosphor for red light represented bya composition of (Ca_(0.985)Eu_(0.015))AlSiN₃, a resin layer thatcontains a phosphor for green light represented by a composition of(Ba_(0.70)Sr_(0.10)Eu_(0.20))(Mg_(0.75)Mn_(0.25))Al₁₀O₁₇, and a resinlayer that contains a phosphor for blue light represented by acomposition of (Ba_(0.80)Eu_(0.20))MgAl₁₀O₁₇.

The light-emitting device of Comparative Example 10 prepared forcomparison with Example 10 differs from that of Example 10 only in thatthere was used a single resin layer which contains a mixture of aphosphor for blue light represented by a composition of(Ba_(0.80)Eu_(0.20))MgAl₁₀O₁₇ and a phosphor for yellow lightrepresented by a composition of (Y_(0.52)Gd_(0.35)Ce_(0.13))₃Al₅O₁₂.

In Example 10 and Comparative Example 10 also, similarly as in the caseof Example 9, the light-emitting devices as shown in FIG. 1 werefabricated and their properties were evaluated. Table 5 shows theevaluation results.

TABLE 5 Brightness Color reproducibility (relative value) Tc-duv (NTSCratio) Ex. 10 100.0% 7700 K−0.001 83.8% Com. Ex. 10 96.8% 7700 K−0.00147.9%

As seen from Table 5, the light-emitting device of Example 10 is alsoconsiderably improved not only in brightness but also in colorreproducibility (NTSC ratio) as compared to Comparative Example 10.

Light-emitting devices of Examples 11-15 of the present invention andComparative Examples 11-15 for comparison therewith were fabricatedsimilarly as in Example 9 and Comparative Example 9. Evaluation resultsof their properties are shown in Table 6. In Examples 11-15 andComparative Examples 11-15, the light-emitting devices as shown in FIG.1 were fabricated, varying the emission peak wavelengths of thelight-emitting elements and the compositions of the phosphors used.

TABLE 6 Peak wavelength Brightness Color reproducibility of primarylight Phosphors (relative value) Tc-duv (NTSC ratio) Ex. 11 425 nm red:(Ca_(0.97)Ba_(0.01)Eu_(0.02))(Al_(0.99)In_(0.01))SiN₃ 100.0% 9000K−0.002 85.2% green:(Ba_(0.50)Sr_(0.35)Eu_(0.15))(Mg_(0.80)Mn_(0.20))Al₁₀O₁₇ blue:(Sr_(0.64)Ba_(0.30)Ca_(0.05)Eu_(0.01))₁₀(PO₄)₆•Cl₂ Com. ″ blue:(Sr_(0.64)Ba_(0.30)Ca_(0.05)Eu_(0.01))₁₀(PO₄)₆•Cl₂ 97.2% 9000 K−0.00248.3% Ex. 11 yellow: (Sr_(0.55)Ba_(0.13)Ca_(0.01)Eu_(0.01))₂SiO₄ Ex. 12400 nm red: (Ca_(0.94)Sr_(0.05)Eu_(0.01))AlSiN₃ 100.0% 8300 K+0.00285.3% green: (Ba_(0.80)Sr_(0.05)Eu_(0.15))(Mg_(0.80)Mn_(0.20))Al₁₀O₁₇blue: (Ba_(0.86)Eu_(0.14))(Mg_(0.99)Mn_(0.01))Al₁₀O₁₇ Com. ″ blue:(Ba_(0.86)Eu_(0.14))(Mg_(0.99)Mn_(0.01))Al₁₀O₁₇ 96.7% 8300 K+0.002 47.8%Ex. 12 yellow: (Y_(0.62)Gd_(0.25)Ce_(0.13))₃(Al_(0.90)Ga_(0.10))₅O₁₂ Ex.13 405 nm red: (Ca_(0.99)Eu_(0.01))(Al_(0.90)Ga_(0.10))SiN₃ 100.0% 6500K−0.002 84.8% green:(Ba_(0.84)Sr_(0.01)Eu_(0.15))(Mg_(0.75)Mn_(0.25))Al₁₀O₁₇ blue:(Ba_(0.82)Sr_(0.03)Eu_(0.15))MgAl₁₀O₁₇ Com. ″ blue:(Ba_(0.82)Sr_(0.03)Eu_(0.15))MgAl₁₀O₁₇ 97.1% 6500 K−0.002 48.2% Ex. 13yellow: (Sr_(0.77)Ba_(0.20)Ca_(0.01)Eu_(0.02))₂SiO₄ Ex. 14 430 nm red:(Ca_(0.99)Eu_(0.01))AlSiN₃ 100.0% 7500 K+0.001 84.9% green:(Ba_(0.45)Sr_(0.35)Eu_(0.20))(Mg_(0.80)Mn_(0.20))Al₁₀O₁₇ blue:(Sr_(0.62)Ba_(0.35)Ca_(0.01)Eu_(0.02))₁₀(PO₄)₆•Cl₂ Com. ″ blue:(Sr_(0.62)Ba_(0.35)Ca_(0.01)Eu_(0.02))₁₀(PO₄)₆•Cl₂ 96.6% 7500 K+0.00148.1% Ex. 14 yellow: (Sr_(0.61)Ba_(0.35)Ca_(0.03)Eu_(0.01))₂SiO₄ Ex. 15395 nm red: (Ca_(0.98)Eu_(0.02))AlSiN₃ 100.0% 8100 K−0.001 85.1% green:(Ba_(0.75)Sr_(0.15)Eu_(0.10))(Mg_(0.75)Mn_(0.25))Al₁₀O₁₇ blue:(Ba_(0.75)Sr_(0.15)Eu_(0.10))MgAl₁₀O₁₇ Com. ″ blue:(Ba_(0.75)Sr_(0.15)Eu_(0.10))MgAl₁₀O₁₇ 97.0% 8100 K−0.001 47.7% Ex. 15yellow: (Y_(0.62)Gd_(0.25)Ce_(0.13))₃(Al_(0.80)Ga_(0.20))₅O₁₂

As seen from Table 6, the light-emitting devices of Examples 11-15containing the phosphors according to the present invention areconsiderably improved not only in brightness but also in colorreproducibility (NTSC ratio), as compared to Comparative Examples 11-15each including a combination of a blue light-emitting element with adivalent europium-activated (Sr,Ba,Ca)₂SiO₄ phosphor or a trivalentcerium-activated (Y,Gd)₃(Al,Ga)₅O₁₂ phosphor that is excited by the bluelight and emits yellow light.

The present invention requires that the peak wavelength of the primarylight emitted from the light-emitting element falls within the rangefrom 380 nm to 450 nm. The light-emitting element having the peakwavelength within the range from 390 nm to 420 nm is more suitable forthe light-emitting device of the present invention.

In order to significantly improve the color reproducibility (NTSCratio), it is desirable to narrow the half bandwidth of emissionspectrum of the phosphor for emitting green light (or components foremitting green light contained therein). To this end, light emission dueto divalent manganese (Mn) as in the present invention is suitable, andthen aluminate is suitable for the matrix of the phosphor for emittinggreen light.

One of features of the above-described Examples is that the resin layersare arranged from the side closer to the light-emitting element indecreasing order of the wavelength of the secondary light emitted by thephosphors contained therein. In the case that a single resin layercontaining a mixture of such phosphors for three colors as in thepresent invention is used for the wavelength conversion unit, however,the color reproducibility (NTSC ratio) is not adversely affected, thoughthe brightness is reduced considerably. Thus, such a single resin layercontaining the mixed phosphors of three colors may be used whenimprovement in color reproducibility (NTSC ratio) alone is pursued.

In the above-described examples, the resin layer containing the phosphorfor blue light is stacked on the resin layer containing the phosphor forgreen light. In the phosphor for green light in the present invention,however, intensity of the second light emitted by excitation with theprimary blue light having a wavelength around 450 nm is weak. This meansthat even if the phosphor for green light and the phosphor for bluelight are mixed together and contained in a single resin layer, thewhite light is hardly decreased in brightness, and thus the functionsand effects of the present invention can be maintained.

In FIG. 9, a main part of a light-emitting device according to Example16 of the present invention is illustrated in a schematic vertical crosssectional view. The light-emitting device of FIG. 9 differs from that ofFIG. 1 in that a light-emitting element 11 is mounted at the bottom of abowl-shaped cup 46 having a reflective surface (inner peripheralsurface) made of a white color resin and having no terraces, and in thatlight-emitting element 11 is sealed with a transparent resin 42containing phosphor particles 43, 44 and 45 distributed in a prescribedmanner.

For transparent resin 42, it is possible to use epoxy resin, siliconeresin or the like. At least two kinds of particles selected fromphosphor particles 43 for red light, phosphor particles 44 for greenlight and phosphor particles 45 for blue light are included intransparent resin 42, and these phosphor particles are distributed asbeing separated approximately into layers depending on their kinds.Here, the phosphor particles for emitting secondary light of a shorterwavelength are distributed farther from light-emitting element 11.

More specifically, transparent resin 42 includes large size phosphorparticles 43, medium size phosphor particles 44, and small size phosphorparticles 45, which are separated into layers. In this case, lightemitted from the larger size phosphor particles can be scattered by thesmaller size phosphor particles, thereby becoming uniform radiation.

The light-emitting device as shown in FIG. 9 can be fabricated in thefollowing manner. As described above, the phosphors contained intransparent resin 42 are those of at least two kinds selected fromphosphor particles 43 for red light, phosphor particles 44 for greenlight and phosphor particles 45 for blue light. The phosphor particlesof the different kinds have their particle sizes adjusted such that theydiffer in settling speed in the transparent resin of a liquid phasebefore being set.

The settling speed of the phosphor particles in the liquid resin isdetermined according to the gravity acting on the phosphor particles andthe magnitude of friction force due to the liquid resin in contact withthe surfaces of the particles. The gravity is proportional to cubic ofthe particle size, and the friction force is proportional to square ofthe particle size. Thus, the particle size considerably affects thesettling speed of the particles. The friction force due to the liquidresin does not much depends on the kind of resin, but mainly depends onthe surface state of the phosphor particle and thus varies depending onthe material and the surface treatment of the phosphor.

In the case of the phosphor particles for which no special surfacetreatment has been performed, the surface area per unit mass isgenerally greater in the finer particles, of which settling speed isslower than that of the coarser particles (see WO 02/059982 A1).Incidentally, in the case that primary phosphor particles coagulate toform secondary particles, the settling speed is determined not dependingon the size of the primary particle but depending on the diameter of thesecondary particle. It is considered that the above-described ideagenerally holds, although the actual phosphor particles are not of anideal sphere shape and simple comparison cannot be done.

To cause the particles distributed in the liquid resin to be separateddepending on their sizes by utilizing the difference in settling speedas described above, it is practically desirable to use phosphorparticles of inorganic material having sizes in the order of several μM.For example, if the particle size is decreased to the order of Bohrradius as disclosed in Japanese Patent Laying-Open No. 2004-071357, theseparation will take a long time, which is not practical.

In Example 16, it is possible to use the phosphor particles for red,green and blue lights having their respective median sizes of, e.g., 13μm, 9.5 μm and 6.5 μm. Here, the median size refers to the central valuein distribution of the particle size. The averaged particle diameter d₅₀is often used as a parameter indicating the particle size as well.Whether the former or the latter is used, not much influence is causedon the effects of the present invention. Incidentally, it is needless tosay that narrower distribution of the particle size in each kind of thephosphors is more preferable from the standpoint of clearer separationbetween the different kinds of phosphors.

More specifically, referring again to FIG. 9, different kinds ofphosphor particles 43, 44 and 45 having different settling speeds arekneaded together into transparent resin 42 of a liquid phase, which isintroduced into cup 46 provided with light-emitting element 11.Thereafter, transparent resin 42 containing phosphor particles 43, 44and 45 is left at rest for a prescribed time period. Then, phosphorparticles 43 having a faster settling speed are distributed such thatthe concentration thereof is higher near the bottom of cup 46 anddecreases as the distance from the bottom increases. In contrast, thephosphor particles 45 having a slower settling speed are distributedsuch that the concentration thereof is lower near the bottom of cup 46and increases as the distance from the bottom increases. As such, theconcentration distribution of the phosphor particles can be formeddepending on the settling speeds thereof.

It is noted that the chemical formulae representing the phosphors andcomposition ratios thereof shown in the above Examples are merelyrepresentative, and that the effects of the present invention can beachieved as long as the phosphors satisfy the compositions andcomposition ratios shown in the “Summary of the Invention” sectionabove.

As described above, the present invention can provide a light-emittingdevice that includes a wavelength conversion unit containing pluralkinds of phosphors for efficiently emitting light in response to theultraviolet light or blue light emitted from a light-emitting element,in which it is easy to set the color of emission light and it ispossible to provides high luminance. Further, it is possible to providea light-emitting device that can radiate white light excellent in colorreproducibility (NTSC ratio).

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

1-24. (canceled)
 25. A light-emitting device, comprising: alight-emitting element for emitting primary light; and a wavelengthconversion unit for absorbing part of the primary light and emittingsecondary light having a wavelength longer than that of the primarylight, wherein said wavelength conversion unit includes plural kinds ofphosphors different in light absorption characteristic from each other,at least one kind of phosphor among said plural kinds of phosphors hasan absorption characteristic that can absorb the secondary light emittedby at least another kind of phosphor among said plural kinds ofphosphors, and said plural kinds of phosphors have median sizesdifferent from each other, and by mixing wavelength components of theprimary light emitted from said light-emitting element and wavelengthcomponents of the secondary light emitted from said plural kinds ofphosphors, the light-emitting device emits light at a chromaticityregarded as blackbody radiation at a prescribed color temperature. 26.The light-emitting device according to claim 25, wherein among saidplural kinds of phosphors, a kind of phosphor for emitting the secondarylight of a relatively longer wavelength has a larger median size thanthat of a kind of phosphor for emitting the secondary light of arelatively shorter wavelength.
 27. The light-emitting device accordingto claim 25, wherein said light-emitting element is formed of a galliumnitride-based semiconductor, and said primary light has a peakwavelength in the range from 380 nm to 500 nm.
 28. The light-emittingdevice according to claim 27, wherein said wavelength conversion unitincludes a combination of a phosphor for emitting blue light, a phosphorfor emitting green light and a phosphor for emitting red light, or acombination of a phosphor for emitting green light and a phosphor foremitting red light.
 29. The light-emitting device according to claim 28,wherein said phosphor for emitting blue light includes at least one kindselected from a divalent europium-activated halophosphate phosphorrepresented by (M1,Eu)₁₀(PO₄)₆.Cl₂, a divalent europium-activatedaluminate phosphor represented by a(M2,Eu)O.bAl₂O₃, and a divalenteuropium-manganese co-activated aluminate phosphor represented by a(M2,Eu_(c),Mn_(d))O.bAl₂O₃, where M1 represents at least one kind ofelement selected from Mg, Ca, Sr and Ba; M2 represents at least one kindof element selected from Mg, Ca, Sr, Ba and Zn; and a, b, c and drepresent numbers satisfying a>0, b>0, 0.1≦a/b≦1.0, and 0.001≦d/c≦0.2.30. The light-emitting device according to claim 28, wherein saidphosphor for emitting green light includes at least one kind selectedfrom a divalent europium-manganese co-activated aluminate phosphorrepresented by a(M2,Eu_(e),Mn_(f))O.bAl₂O₃, a divalenteuropium-activated silicate phosphor represented by2(M1_(1-g),Eu_(g))O.SiO₂, and a trivalent cerium-activated silicatephosphor represented by MI₃(MII_(1-h),Ce_(h))₂(SiO₄)₃, where M2represents at least one kind of element selected from Mg, Ca, Sr, Ba andZn; a, b, e, and f represent numbers satisfying a>0, b>0, 0.1≦a/b≦1.0,and 0.3≦f/e≦5.0; M1 represents at least one kind of element selectedfrom Mg, Ca, Sr and Ba; g represents a number satisfying 0.005≦g≦0.10;MI represents at least one kind of element selected from Mg, Ca, Sr andBa; MII represents at least one kind of element selected from Al, Ga,In, Sc, Y, La, Gd and Lu; and h represents a number satisfying0.01≦h≦0.4.
 31. The light-emitting device according to claim 28, whereinsaid phosphor for emitting red light includes a divalenteuropium-activated nitride phosphor represented by(MIII_(1-j),Eu_(j))MIVSiN₃, where MIII represents at least one kind ofelement selected from Mg, Ca, Sr and Ba, MIV represents at least onekind of element selected from Al, Ga, In, Sc, Y, La, Gd and Lu, and jrepresents a number satisfying 0.001≦j≦0.05.
 32. The light-emittingdevice according to claim 28, wherein said light-emitting element iscovered with a plurality of resin layers, each of said resin layersincluding one of said phosphor for red light, said phosphor for greenlight and said phosphor for blue light, and then the phosphor containedin the resin layer closer to said light-emitting element emits thesecondary light having a longer peak wavelength as compared to thephosphor contained in the resin layer farther from said light-emittingelement.
 33. The light-emitting device according to claim 29, wherein anoptical film for transmitting said secondary light and reflecting onlysaid primary light selectively depending on their wavelengths isprovided between an outermost surface of said wavelength conversion unitand an exterior space.
 34. A fabrication method of a light-emittingdevice, comprising the steps of: mounting a light-emitting element on abottom surrounded by a reflective surface of an inner wall in a package;introducing a liquid resin kneaded with plural kinds of phosphors havingdifferent median values of particle sizes, to cover said light-emittingelement mounted in said package; and leaving said introduced liquidresin at rest for a prescribed time period to cause the phosphorparticles to settle such that a particular kind of the phosphor having agreater median value of the particle size has a greater concentrationnear said light-emitting element.